Habitable? There is plenty of water for it to be habitable, residential use is a drop in the bucket compared with agricultural use. Without a technological solution, we'll have to scale back agriculture, but that's not a regional problem, since the market for food is an international one.
Also, we don't have to get all of the water from desalination. Southern California can use desalination, which makes sense to the ocean, short distances, and solar power. That frees up Colorado River water that Nevada and Arizona can use.
Desalination will probably never be worth it from agriculture, but agriculture should survive on the water they can access after people.
